Safety limit-switch.



D. R. KNAPP.

SAFETY LIMIT SWITCH.

APPLICATIQN FILED 050.23. 1911.

1 299 839. Patented Apr. 8,1919.

' 2 SHEETS-SHEET l- D. R. KNAPP.

SAFETY LIMIT SWITCH.

APPLICATION FILED DEC.28, 19w.

1,299,839. I Patented Apf. 8, 1919.

2 SHEETS-SHEET 2.

Jiw /0 z W 42 I field 'fi a/a /4 Imwiom DAVID R. KNAPP, OF POTTSVILLE, PENNSYLVANIA.

Specification of Letters Patent.

Patented 'Apr. 8, 1919.

Application filed December 28, 1917. Serial No. 209,277.

To all whom it may concern:

State of Pennsylvania, have invented certain new and useful Improvements in Safety Limit-Switches, of which the following is a specification.

My invention relates to that class of devices used on electric operated hoists wherein it is desired to automatically stop the hoist after the hook or block has reached a posi-. tion. that a further movement would engage the hook or block against the hoisting drum, thereby damaging the hoisting structure or breaking the hoistin rope and impairing the safety of the worimen from the falling material. I

On account of restrictedv head room or travel of the hoisting hook, in sonle cases it is necessary for the hook to raise to a point very close to the drum and in other cases the limit of travel can be as far as several feet from the drum. In either case there is danger of the power being still applied after the hook has reached this danger position, due to an inoperative controller, carelessness or many other causes, which obviously results in considerable damage and sometimes loss of life if the ropes should rupture and allow the parts to fall.

In some forms of safety limit switches the circuit is opened by some means operative withthe hoist and the switch is reset'by hand. This is objectionable due to the fact that the operator may have to climb considerable distance or that the position that the machinery is in maysubject him to great danger or that the loss of time required may be of ital importance.

It is therefore the object-of my invention to provide; first, a switching mechanism that will provide circuits to automatically stop the motor after the hook or block has raised to a predetermined height; second, to provide an electric dynamic circuit on the armature when the switching mechanism is operated to insure a quick stop in case the danger limit is set close to the drum; and third, to provide a means to close the switch and lower the block from the operators controller, andfourth, to provide a limit switch that will not 0' en the motor circuit when 0 base upon WhlOll the swltch is mounted.

operated, there y eliminating destructive arcs on the switch.

\Vith these objects in view my invention consists in certain novel features of construction and arrangement of parts, as will hereinafter be more fully described and pointed out in the claims, reference being had to the accompanying drawings forming part thereof, and in which; Figure l is an elevation showing the device as applied to a hoisting drum equipped with a cable and a hook. Fig. 2 is a plan view of the switch. Figs. 3 and 4 are views'showing in detail the contact brush. Figs. 5, 6, 7 and 8 are views showing details of the brush holding device on the extremities of the movable arm. Figs. 9, 1O, 11 and 12 are views showing in detail the brushholding device fixed on the base of the switch. Fig. 13 is a plan View of the weight showing more clearly the eye bolts and the cable within. Fig. 14. is a wiring diagram showing the method of wiring theswitch to a standard reversing drum' controller used on hoists so as to provide a dynamic stopping circuit and also a circuitfor lowering after the switch has operated.

In a case where the controller 50 is in movable relation to the motor and switch as where the device is employed with an electric traveling crane having the 'control cab fixed to the crane girders, it is, of course necessary to empldy trolley wires connected by suitable leads to the movable parts of the "circuits. In the present invention, portions of the leads 52, 55, ,56 and positive lead to brake coil are utilized as trolley wires therefor, making it unnecessary to add any wires to the ordinary trolley wires employed indevices of this kind wherein four are required to operate the hoist without the switch. v

Referring to the figures in the drawings, similar numerals of reference refer to similar parts throughout the several views, and in which the numeral 1 designates a hoisting mechanism, usually 'a drum provided with the necessary motive power, and said drum is provided with a rope 3 and at the extremity thereof is attached the hook 2 by which the material is fastened to be lifted. It desired therefore that when the hook 2 is a predetermined distance from the drum 5 designates a pivot supporting casting and is fulcrume for a hereinafter mentioned switch blade and said casting is secured to said base 4 by the screw 6. V

The numeral 38 designates a binding post forfan electrical connection. 7 designates a switch blade su ported by the casting 5 d thereto by the pin 8. Said switch blade 7 is provided at each extremity'with a contact brush 9 and 10wvhich forms in effect a two point switch. In apparatus of this character the contact brushes require frequent renewing and it is essential that substantial and convenient means of renewal be accomplished. This construction is more clearly shown in Figs- 5 and 6 in which the ends of said switch blade-7 terminates in a protruded lip 40, andthe under portion adapted to form an acute angle with the main portion of the switch arm. 14 designates a clampingbar and is ing' bar 14 which is'securedby the screw 16 passing 'through\ said clamping bar and through the aperture 43 of contact brush 9 and engaging by threaded relation with the switch blade 7 by the threaded aperture 42, clearly shown in Fig. 6.

19 and 20 designate respectively contact brush bases rigidly fixed to said base 4 by the screws 33 and 34. I

Said bases 19 and 20 are provided respectively with clamping bars 18and 17 and with contact'brushes 11 and 12.

- 31 and 32 designate securing screws which engage the contact brush clamping bar in fixed relation similar tolthat heretofore described at the end of the switch blade 7; It

isobvious that the construction discloses a convenient means for securing or removing the contact brushes.

21 designates a yoke pivoted to said switch blade 7 by the pin 35 and 19 designates a rod in threadedengagement with said yoke 21 and passing through an aperture in the base, where .it terminates in an insulator 27, thus insulating the lever parts of the switch from the hoisting mechanism.

28 designates a flexible rope secured to'the insulator 27 by the screw eye 43.

29 designates a weight secured to the rope 28 by any convenient means, and said weight is provided with the screw eyes 30 through which the hoisting rope 3 is adapted to pass freely. It is understood that the weight 29 can be adjusted to any relative distance to the drum by adjusting the length of the rope 28.

contacts 9 and 11, lished as follows:

22 designates a yoke adapted to embrace fulcrum to the yoke 21.

24 designates a rod secured to the yoke 22 by threaded relation and adapted to ass through an aperture in the base an 25 designates a counter weight attached to said rod 24 by threaded relation and made secure thereto by the jam nut 26. The said counter weight is of insufiicient weight to overbala'nce the weight 29 but is of sufiicient weight to close the switch after the hook has raised the weight 29. The numeral 44 designatesthe switch lever 7 at a polnt opposite the terminal lugs integral with the contact brush base 19 and 20, and 37 and 39 designate contact bolts by which terminal leads are fastened.

49 designates apertures in the base 4 for securing to some substantial support above I the hoisting drum 1.

5O designates, diagrammatically, areversing controller adapted to control the hoistin and lowering motion of the hook 2. R designates the control resistance and R designates an auxiliary resistance. The numerals 51, 52, 53, 54, 55, and 56 designate leads interconnecting said controller, safety switch and the various parts.

Having thus described the various parts throughout the several views, all of which will be understood by those skilled in the art to whichthis invention relates, its mode of operation will be substantially as follows:

Assuming that the safety limit switch is mounted in a fixed position over the hoisting drum and that the rope 28 is adjusted in length as to place'the weight 29 in a position where the safety limit switch is to operate.

The weight29 being of suliicient weight to overbalance the counter weight 25, its weight will close the contacts 9 and 11 and open the contacts 10 and 12. This will form the normal raising, and lowering circuits and will be as follows: for lowering, positive line, brake, field, lead 52 to controller, lead 56, contacts 11 and 9, through switch blade, lead 54, armature, lead 55 to controller and thence in the usual manner to negative line.

For the hoisting motion; positive line, brake, through armature in a reverse direction relative to lowering, lead 54, switch blade, contacts 9 and 11, lead 56 to controller and field, lead 52, .to ,controller, lead engaging and lifting the 7 Assume that the control resistance R has been cut out and the hook is hoisting full speed. I

From positive line, brake, field, lead 51, contacts 12 and 10, through switch arm, lead 53, R lead 56 to controller and thence to negative line. It will be seen that this circuit throws the fields and brake across the line with the resistance R- in series and energizes the field.

An additional circuit will be established as follows: armature, lead 55, to controller on the up side, lead 52, lead 51, contacts 12 and 10, through switch arm, lead 54, back to armature. It will be seen that this places the armature on a short circuit.

The armature in such a condition with an energized field will set up a strong dynamic current which will cause the armature and the hoisting drum to come to a quick stop, thus preventing any damage to the hoisting mechanism, thus accomplishing the first and second objects of my said invention.

The controller is now brou ht to the oil position which will set the bra es due to the circuit being opened, the contacts 10 and 12 will still be closed and the contacts 9 and 11 open. The controller is then thrown to the lowering position and a circuit will be'established as follows: positive line, brake, field, lead 51, contacts 12 and 10, through switch blade, lead 54., armature, lead 55 to controller, and thence to line forming a lowering circuit, the current passing through the armature in the same direction as when the switch is in the normal osition. There will also be an additional circuit established as follows: positive line, brake, field, lead 52, to controller, lead 56, R lead 53, and combining with lead 54' from the former circuit from lead 51. This circuit is inactive due to the fact that the previous circuit is shorter and has no inserted resistance.

Upon lowering the hook 2 a slight distance the weight 29 again closes the switch to its normal position as heretofore described, thus accomplishingv the third object of my said invention. It will be noted that the opening and closingof the switch will not be accompanied with any severe are due to the fact that the motor circuit is never opened by the switch contacts.

In some forms of limit switches the motor circuit is broken by the switch which is objectionable due to the fact that a severe arc occurs. In Fig. 14 it will be seen that the current traverses contacts 9 and 11 and if those contacts are opened the current will take a path through the resistance R whichis shunted around these contacts, thereforeno severe arc can take place. It will be seen that before the dynamic'circuit contacts 10 and 12 close,'the contacts 9 and 11 will have opened which will automatically insert resistance in the hoisting circuit which will slow down the motor to a considerable extent before the dynamic circuit is closed, thus accomplishing the fourth object of my said invention.

It will further be apparent that an eifective braking action is accomplished without utilizing the friction brake and also without changing the position of the controller whereby the personal equation is entirely eliminated. .The force of the dynamic effort to stop the motor depends on the strength of the field winding of the motor and the amount of current permitted to flow through the resistance connected across the switch.

but what I claim as'new and desire to secure by Letters Patent is: I

1. In a device of the character stated, the combination of a hoisting mechanism, a hoisting circuit and a lowering circuit for said mechanism, a controller for said circuits, a normally open dynamic brake circuit, an. overhoisting limit switch, means including a suspended weight to automatically operate said switch to close said braking circuit, means arranged to open a portion of the operating hoist circuit upon operation of said switch, and means to maintain the operating hoist circuit closed to prevent arcing during the operation of said switch.

2. In a device of the character stated, the combination of a hoisting mechanism, a hoisting circuit and a lowering circuit for said mechanism, said lowering circuit having two gaps in inoperative condition, a

controller for said circuits,'a normally open dynamic braking circuit, 'a limit switch, means including a suspended weight to automatically operate said switch to close said braking circuit and one gap of said lowering circuit and means to prevent arcing at said switch while the lowering circuit is maintained ready to be effective upon the closure of the other gap of said lowering circuit by theoperation of said controller.

3. In a device of the character stated, the

combination of a hoisting mechanism, a'

hoisting circuit and a lowering circuit for sald mechanism, a controller for said circuits, a brake coll common to said circuits and normally energized, two setsof contacts, a limit switch normally closing one of said. sets of contacts and means operated by the hoisting circuit for said mechanism, a brake coil in said circuit, means automatically operable to retard said hoisting mechanism at a predetermined position and to maintain said hoisting circuit closed and said brake coil energized, an open lowering circuit, means controlled by said automatic means to partially complete saidlowering circuit and a controller for breaking said hoisting circuit and completing said lowering circuit.

5. In a device of the character stated, the combination of a hoisting mechanism, two operating circuits for said hoistingmechanism, one of said circuits being closed to operate said hoisting mechanism in one direction and the other circuit being open, a brake coil common to said circuits, means operable to form-a dynamic braking circuit to maintain said operating circuit closed to maintain said brake coil energized and to partially close said open circuit and a controller for breaking said closed circuit and completing the partially closed circuit.

6. In a device of the character stated, the

combination of a hoisting mechanism, two

operating circuits for said hoisting mechanism, one of said circuits being closed to operate said-hoisting mechanism in one direction and the other circuit then being open,

a brake coil common tcsaid circ'zuits, a set of contacts common to said circuits, a switch in said circuits and controlling said contacts,

means to shift said switch from one contact to the other and a resistance normally in both circuits to prevent the closed operating circuit being broken and to maintain said brake coil energized.

-7. In a device of the character stated, the combination of a hoisting mechanism, two operating circuits for said hoisting mechanism, one of said circuits being closed to operate said hoisting mechanism in one direction and the other circuit then being open, a brake coil common to said, circuits, a set of contacts common to said circuits, a switch in said circuitsand controlling said contacts, means to shift said switchfrom one 7 contact to the other, a resistance normally in both circuits to prevent the closed operating circuit being broken and to maintain said brake coil energized and a controller for breaking either circuit and to change from one circuit tothe other. i

In testimony whereof 'I have signed this specification in the preseneof two subscribing witnesses.

' f DAVID R. KNAPP. Witnesses:

@LATION A. Hat, 4 HAROLD C. Brown, 

